Gas turbine engines have long been used to convert chemical potential energy, in the form of fuel, to thermal energy, and then to mechanical energy for use in propelling aircraft, generating electric power, pumping fluids, etc. The efficiency of gas turbine engines increases with increasing operating temperatures. Therefore, there is a large incentive to raise the combustion and exhaust gas temperatures of such engines. However, the metallic materials currently used in the hot-section components of such engines operate at temperatures very near the upper limits of their thermal stability. In fact, in the hottest sections of modern gas turbine engines, metallic materials are utilized in hot gas paths at temperatures above their melting points. These metallic materials survive such temperatures only because they are air cooled, or because they comprise ceramic coatings thereon that lower the thermal conductivity of the component, thereby allowing the components to be operated at higher temperatures while utilizing less cooling air. Such ceramic coatings may additionally or alternatively provide environmental protection to the metallic components, protecting the components against the oxidative and corrosive effects of the hot gases passing therethrough or thereby.
During engine operation, the airfoil and inner buttress surfaces of the turbine vanes are directly exposed to the hot gases, and are therefore susceptible to accelerated oxidation and corrosion. Accordingly, a ceramic coating is generally applied to the airfoil and inner buttress surfaces of these vanes. In contrast, the remaining surfaces of these vanes are not directly exposed to the hot gases, and coating these remaining surfaces may actually be detrimental and degrade the fatigue life of the attachment mechanisms and other highly stressed regions thereof. Therefore, these remaining surfaces are typically left uncoated.
Fixtures are often used to mask the selected portions of the vane that are not being coated. However, with such fixtures, bridging of the ceramic coating being applied to the vane often occurs between the vane and the fixture itself. This bridging may cause some of the ceramic coating to chip away from the vane, or cause extra ceramic coating to adhere to the vane, when the vane is removed from the fixture. Both cases require a significant amount of handwork and/or rework in order to prepare the vane for use in a gas turbine engine.
It would be desirable to have fixtures for selectively masking turbine vanes or other workpieces prior to coating unmasked portions thereof. It would be further desirable to have fixtures that allowed such workpieces to be coated without bridging of the workpiece coating occurring between the fixture and the workpiece during application of a workpiece coating. It would be particularly desirable to have such fixtures be non-stick with respect to the workpiece coating being deposited on the workpiece so that less handwork and/or rework than currently required would be necessary.